In recent years, technology is rapidly developing in the digital industry and new products different from existing products, such as wearable devices have been released.
There are various kinds of wearable devices such as various types of glasses, wrist watches/wristband units, shoes/insoles, rings, waist belts, arm band units, necklaces, ear sets, clothing, and badges. Smart band units and smart watches worn on the wrist form the largest group of products.
Korean Patent Registration No. 672444 disclosed a watch phone apparatus including: a watch phone having a mobile communication terminal that is implemented in a watch formed; a watch strap that secures the watch phone on a wrist of a user; a first battery that is provided in the watch phone and for supplying power to the watch phone; a second battery that is provided in the watch strap and for supplying power to the watch phone; and a charging terminal that is provided in a portion where the watch strap is tied and for charging at least one of the first and second batteries. However, Korean Patent Registration No. 672444 disclosed that only a lithium ion (Li-ion) or lithium polymer (Li-polymer) battery was applied for the second battery located in the watch strap, but did not specifically present the structure of the second battery.